Then, suddenly, without warning or explanation, ISAIAH.
Nephi spends this entire chapter quoting Isaiah Chapter 48, although he makes a few changes to the wording. The basic idea is that Israel is chosen, and it's had some rough times, but that's helped make it better (refined in the furnace of affliction).
I imagine Joseph Smith included this to give his claims more credibility. Of course it's a true Christian faith--look, it quotes the Bible! If that's not proof that the two books come from exactly the same source, what is? Never mind that it seems that Nephi didn't need to record any of the Old Testament for us (Joseph Smith: A Master of Textual Efficiency).
It's also possible that he temporarily ran out of ideas. The Isaiah-quoting passages are pretty much filler anyway. Maybe he was just buying time while he thought of an interesting direction for the story to take now that the big journey to the promised land is over with.